Methods for detecting specific acoustic spectral features are described in U.S. Pat. Nos. 4,117,731; 3,952,578; 4,383,446; 4,167,879; 3,572,099 and 3,290,922. The acoustic methods in these patents are all based on the frequency-sweeping of a single instantaneous frequency acting on a sample and examining the transmitted or scattered acoustic signal which is of nearly the same shape as the incident signal. As shown in FIG. 1, an incident beam 10 in accordance with these prior art acoustic methods having a waveform shape 12 is directed at a sample 14. The beam 16 emerging from the sample 14 has a waveform 18. The instantaneous Fourier spectra 20 and 22 of the instant beam 10 and transmitted beam 16, respectively are also shown. P(t) is the field variation with time 5, while I(f) is the Fourier intensity at frequency f. The problem with these acoustic methods is that the intensity of the waveform is smaller than and the frequency of the transmitted beam is the same as that of the incident beam. As a result, these methods are not as accurate and precise as desired.
The patent Bjorklund, U.S. Pat. No. 4,297,035, describes an optical method which uses a beam which is modulated with a single RF frequency to provide a pure FM spectrum having upper and lower sidebands. The sample is exposed to the modulated beam so that only one of the FM sidebands probes the narrow spectral feature. The light emerging from the sample is photodetected to provide a RF beat. The amplitude of this RF beat is monitored to indicate the strength of the narrow spectral feature. This optical method deals with optical parameters which are of an altogether different magnitude from acoustic parameters. For example, the optical method involves a wavelength of 10.sup.-6 meters whereas an acoustic method is concerned with wavelengths of 10 meters to 10.sup.-5 meters. Hence, acoustic wavelengths may be 40,000 times larger than the optical wavelengths and diffraction effects would be much stronger. The optical method wavespeed is 3.times.10.sup.8 meter per second, whereas an acoustic wavespeed is 3.times.10.sup.2 meter per second. Since the acoustic wavespeed is slower by one million times it would be reasonable to expect excessive signal temporal spread to occur. The optical method probes molecules having sizes of 10.sup.-10 meters whereas an acoustic method probes acoustic absorbers having sizes of 10 meters to 10.sup.-5 meters. As a result of this larger size of the active species, the coherent effect may not be observable for wave scattering from large objects. The optical method is concerned with vibrating electromagnetic fields propagating in space whereas the acoustic method deals with compression and rarefaction fluctuations in materials.